hibernate.md

Hibernate

Working with SQL statements and ResultSets can be cumbersome:

• There could be multiple places in the app where you transform between SQL columns and classes. If a column changes you'd need to update all the places.
• SQL statements themselves could be pretty large and complicated especially when you need to fetch many related entities

ORM (Object-Relational Mapping) tools allow to map the fields of the classes to the DB columns and then will generate SQL statements for you. Which is awesome since now you can concentrate on the business logic and OOP instead of DB.

This also gives you a possibility to switch between DBMS easily since you don't write native SQLs anymore which could differ from DBMS to DBMS. Such feature is very convenient because it's possible to write DAO tests that could be run on in-memory DB instead of the real one. Of course in our case H2 is already an in-memory DB, but this is a very rare case in real life.

Additionally ORMs try to optimize the work with DB in different ways:

• They delay the actual execution of SQL as much as possible. This decreases the amount of time the rows and tables are locked.
• They can (if configured) group INSERT and UPDATE statements together and issue bulk statements instead of executing them one by one.
• They can transparently use process-level cache that will prevent some of the SELECT statements from happening.

Dark Side of ORM

But ORMs have issues as well:

• They are very complicated. While working with DB itself is not trivial, ORMs add yet another layer of complexity which makes it challenging for many people. So while they may simplify the app, to use them effectively you should know a lot.
• Not optimal SQL. Since you create one mapping and then re-use it in many places the mapping can be optimal not for every place. Sometimes you just need to select one column, other times you need to update only one column. ORMs don't allow this - they work with objects and their hierarchy.

So instead of using ORMs as a universal solution you should understand where it's more applicable. You can start at the beginning of the project with ORM since a lot of changes and refactorings are expected at that stage. And then if the app is being developed long enough you'll probably start moving towards native SQL.

Hibernate vs JPA

Among different ORMs historically Hibernate won the battle and became the most used one. Its success impacted the way further ORMs were implemented in Java community. Hibernate developers participated in the development of JPA (Java Persistence API) standard and effectively made Hibernate its reference implementation. So now we have different ways of working with Hibernate:

• Hibernate XML
• Hibernate Annotations - most of these were deprecated in favour of JPA Annotations but some are still in use when a more fine-grained control is required.
• JPA Annotations
• JPA XML (haven't seen these in practice at all)

All these approaches do the same and they operate with the same basic principles so if you know one it's not hard to switch to another. The terminology though could be different. E.g. you can work with things like Session, Components when you work with Hibernate, but they are going to be called EntityManager and Embedded Object in JPA.

JPA Annotations currently is the most widespread way of working with Hibernate. My personal preference though is Hibernate XML:

• This is the most functionally-rich way of mapping
• It doesn't bloat Entity classes with lots of persistence annotations

Step 1 - How it works

• Read about Hibernate Proxies
• Read about Entity Lifecycle in Hibernate (PERSISTED, DETACHED, etc)
• Read about Identity Generators (ways to generate IDs). Which one would you prefer and why? Which of them require communication with DB and which don't?
• Add Hibernate dependencies and look into classes SessionImpl and StatefulPersistenceContext

Step 2 - Re-write DAO

• Implement an alternative implementation of DAO.
  • Instead of re-writing the previous one, create interfaces that are implemented both by JDBC DAO and by Hibernate DAO. It should be possible to switch between DAO implementations between startups via Spring means (there are multiple ways of doing this - which one do you choose?).
  • Choose appropriate Spring's TransactionManager for Hibernate
• Implement Hibernate Dao Tests - they need to import spring context and start the embedded H2 DB
• Play with get() vs load() methods and explore in debug how Hibernate Proxy looks

Step 3 - Persisting Entities

• Read about Flush Mode. When in the app do you think the SQL statements are flushed? Configure SQL logging and check your guess.
• After the entity is persisted it must have an ID. Return to the ID Generation Strategies and find those that require INSERT statements for the ID to be generated. Will Flush Mode impact these INSERT statements in any way? Would that change if you choose a different strategy?
• Research what Bulk Updates do - how can this improve the performance?

Step 4 - Loading Data

• Read about lazy loading. How can it work for x-to-one associations? How can it work for collections?
• Create another class, map it and put a collection of it in Dog. Play with fetch and lazy attributes - which of them allows for lazy loading and which don't?
• Make the collection lazy. Load the main entity, then close the session. Now try to access the lazy connection.
• Given the collection is lazy, how to override this setting with HQL?
• Research what Batch Select means and how it can improve the performance

Alternatives

While Hibernate is a full blown ORM that both generates SQL statements and keeps track of Persisted entities, there are some lightweight alternatives. They implement ORM only partially and therefore move the balance between low-level and high-level control over the DB-related code. Take a look at these: Spring JDBC (we've touched it already), JOOQ, MyBatis.

Another framework to mention is Spring Data. It's an even higher level of abstraction than JPA - you use the very same mapping, but instead of manually creating EntityManager/Session, creating Query object and casting the results you simply create interfaces and mark them with the appropriate annotations. And the rest is done for you. But before rushing into using it remember - the higher the level of abstraction the more you have to know and the slower it works.

Puzzles

• There is a bug in Hibernate mapping in branch puzzle-wG3JaxG4. But the test actually passes. Find the reason and fix the test.
• Next puzzle is in branch puzzle-TsomSH6a. There is a failing test that checks that if we violate a DB constraint, an exception is raised. But that doesn't happen. Looks like the DB constraints don't work. Or do they?
• Another one: puzzle-BS7MrHNE. There is a failing test - our Dog cannot find its owner, you need to help it.

Literature

• POJOs in Action - one of the fundamental books on ORMs. Explains the principles instead of just going through the feature list of ORMs.
• Java Persistence with Hibernate - a deep book that goes over the features of Hibernate and the reasons it works that way. It's better to be read after POJOs in Action as it's pretty complicated.